Fourteen gnotobiotic calves were killed 0.5 to ten days after infection with Newbury agent SRV-1 and the changes in small intestinal structure and function were assessed, qualitatively and quantitatively, by light microscopy, scanning and transmission electron microscopy, enzymology and xylose absorption. The first enterocytes detected as infected by immunoperoxidase were those on the sides of villi at the base. Subsequently exfoliation of degenerate enterocytes resulted in stunted villi; mucosal beta-galactosidase activity fell and there was xylose malabsorption. Small intestinal damage, first detected at 12 hours after infection but almost repaired by ten days, was restricted to the anterior half of the small intestine. In the distal small intestine, where no virus-induced damage occurred, villi lengthened--possibly due to increased mitosis of crypt cells stimulated by enteroglucagon release.